Azo dyes

The azo dyes constitute the largest chemical class, containing at least 66% of all colorants.  The characteristic feature is the presence in the structures of one or more azo groups,

together with hydroxyl groups, amine and substituted amine groups as auxochromes.

Aromatic azo compounds are produced from aromatic amines via the corresponding diazonium salt.

A diazonium salt is formed when an aromatic amine is treated with nitrous (nitric(III)) acid.  The nitrous acid is formed in situ by adding dilute hydrochloric acid to a cool solution of sodium nitrite at ca 278 K.  In the following example, a solution of benzenediazonium chloride has been formed from phenylamine (aniline), the simplest aromatic amine:

 

A solution of another compound such as another aromatic amine or a phenol is then added to the cool solution and produces an azo compound which is coloured.  One example is the formation of a red dye when an aqueous solution of 4-aminonaphthalenesulfonic acid (naphthionic acid) is added to a solution of 4-nitrobenzenediazonium chloride to form C.I. Acid Red 74:

Azobenzene is the chromophore of these azo dyes,

and the colour of the molecule can be modified and the intensity of colour increased by varying the auxochromes (Table 3).

Structure	Colour observed
	yellow-green
	yellow
	red
	blue

Table 3  The molecular structures of some azo dyes showing the auxochromes.

Some azo dyes, those containing a hydroxy group ortho (or para) to the azo group, for example, C.I. Acid Orange 7, exhibit tautomerism, a process in which the molecule exists as two or more different structures in equilibrium.  The hydrogen atom on the hydroxyl group is able to migrate to the nitrogen atom of the azo group and vice versa:

This type of tautomerism involves an equilibrium between a hydroxyazo tautomer and a ketohydrazone tautomer, although the ketohydrazone tautomer generally dominates and the colour observed is of longer wavelength (a bathochromic shift).